(1) Field of the Invention
The present invention relates to a color image processing unit for processing images such as color photographs and color pictures and, more particularly, to a method for airbrushing in order to gradate a designated region or regions of color image data and a device therefor.
(2) Description of the Related Art
In recent years, color image processing units for handling color images by means of a computer have been developed in order to answer the growing demand for colored printing in the field of printing, newspaper and design industries. One technique of image processing which is used in conjunction with color processing units is known as "airbrush". Airbrush is a process to gradate a designated region or regions of image data as if color was sprayed on the image data. Airbrush is utilized for a variety of operations such as, for example, connecting two or more images together or superposing letters on an airbrushed region of an image.
In the prior art, airbrush operations have been commonly performed on the color image data which were represented in the RGB coordinate system or in the YMC coordinate system. A conventional method for airbrushing of this kind is shown in FIGS. 13(a), 13(b) and 13 (c).
First, an airbrush operation is performed on image data R.sub.k G.sub.k B.sub.k (k=1, n) in an airbrush effective region of RGB data which is input through a scanner by means of brush colors (RGB) and brush sizes, as designated by an operator (see FIG. 13(a)). The RGB data is airbrushed at a designated brush % of a predetermined designated brush curve as shown in FIG. 13(b).
Then, R,.sub.k G,.sub.k B,.sub.k (k=1, n), as a result of the above airbrushing at a designated brush %, is calculated. Density of the RGB data, as a result of the airbrushing, is illustrated along a particular line in the direction of the X-axis in FIG. 13(c). The result of the airbrush operation is expressed in dotted lines.
However, the conventional airbrush process performed in the RGB space or YMC space often causes, in part, a sudden change of brightness or a shift of color even when the brush % is decreased linearly from the center of the brush towards the brush frames as shown in FIG. 14(a). This is because the brush %, in view of human eyes, decreases non-linearly from the center of the brush towards the brush frames as shown in FIG. 14(b). In other words, the RGB space does not correspond to human senses. Accordingly, it was necessary to draw a brush curve as shown in FIG. 14(c) so that the brush % in view of human eyes forms a brush curve as shown in FIG. 14(a). Hence the operator's job was made very difficult and troublesome.
Further, in the RGB or YMC space, even if attempts are made to vary lightness (brightness) and vividness or chroma, variations of R, G, B (Y, M, C) do not correspond to lightness and chroma. It is thus necessary for each color to seek different brush colors with different lightness and/or chroma prior to the processing, thereby making the operations more difficult and complicated.
The present invention responds to the aforesaid problems, and its purpose is to provide a method for airbrushing and a device therefor, which enable an operator to perform an airbrush operation without a need to adjust a brush curve, and also to perform an airbrush operation in which only lightness or chroma is varied.